You know this world? This sad, doomed little world? It needs you. It needs every last Winchester it can get, and I will not let you die. I wont let any of you die. And I wont let you sacrifice yourselves, you mean too much to me! To everything! Yeah, you made a deal. You made a stupid deal, and I broke it. Youre welcome.
Castiel is an angel. He manifests on Earth inside his vessel, Jimmy Novak. He is the only angel that is shown to have been resurrected after having been killed, and he has been resurrected more than once.
Many angels laid siege to Hell to rescue Dean, but it was Castiel who ultimately pulled Dean out. According to Castiel, he did so "Because God commanded it." His hand print was burned into Deans left shoulder. Castiel is amongst a group of angels who manifest on Earth to stop the breaking of the the 66 Seals which will release Lucifer and start the Apocalypse. Over time he develops an empathy for the Winchesters, in particular Dean. When he learns that Michael actually intends for Lucifer to be released so he can kill him, Castiel tries to warn Dean, but is punished. Ultimately he breaks allegiance with Heaven to side with humanity.

With Lucifer free, and Castiel cut off from Heavens power, he decides to try and find the long absent God. When he learns God is not interested in intervening to stop the Apocalypse, he is devastated. He works to support the efforts of Sam, Dean and Bobby in stopping the battle between Michael and Lucifer.
After Sams sacrifice to defeat Lucifer, Castiel tries to rescue him from Lucifers Cage, but fails to retrieve Sams soul. In an attempt to prevent Raphael from restarting the Apocalypse, Castiel is tempted into an alliance with Crowley. The demon offers to help find Purgatory which can provide them both with great power. His efforts mean Castiel becomes increasingly distanced from the Winchesters as he hides his efforts from them, to the extent of breaking Sams mind in order to hinder their pursuit of him.
After taking in the souls of Purgatory, Castiel kills Raphael. He claims that he is no longer an angel, but the new "God." After smiting many people he considered to be doing evil in Gods name, he blacks out and loses control of his vessel, and his powers, to Leviathans possessing him. Conceding to Dean and Sams wishes, Castiel attempts to return all the souls and entities inside him to Purgatory.
After the souls leave Castiel, he apologizes to Dean, and pledges to find some way of redeeming himself. His plans are cut short when the Leviathans, who managed to hold on, take over his vessel. As the vessel rapidly deteriorates, the Leviathans march it into a reservoir, where it disappears beneath the water, releasing them.
Castiel is later found wandering naked in the woods near the water with no memory of who he is by Daphne Allen, who claims that God led her to him. She takes him home, he takes on the name Emmanuel Allen, and they marry. He discovers he has a gift for healing.
While searching for a way to help Sam, whose psychosis is killing him, Dean hears about Emmanuel from another hunter, and is shocked to discover it is Castiel. Castiel eventually regains his memories, and is horrified at what he did. He tries to heal Sam, but unable to do so, he chooses to take Sams madness into his own mind.